This invention relates to a single pass color printer and, more particularly, to a printer which uses a plurality of laser beams applied to a common scanning mechanism and directed therefrom onto separate exposure areas of a plurality of photoreceptor drums.
Reproducing or copying color originals through a xerographic process has, in the past, entailed the sequential production of three color separation images of the colored original, with independent development thereof by cyan, magenta and yellow toners. The images so formed are transferred onto the copy substrate material in registered overlaying relationship, with the resulting composite color image being fused to provide a permanent full color reproduction of the original.
In the aforedescribed color process, black is obtained through an amalgam of the three color toners. However, it is often useful to provide a separate processing unit devoted solely to black. Because of the need to process three and possibly four color separation images for each copy, the copy output is often very low. Where a single photoconductive drum is used, for example, normally each color separation image is created, developed and transferred to the copy substrate material before the next is started.
In order to increase the throughput of printers producing multilayer images, single pass, multiple exposure systems have been developed and are known in the art. There are a variety of methods known for creating multiple ROS single pass color printer architectures. A desirable configuration, because of the high throughput, is the tandem system where multiple exposures are simultaneously produced on separate photoreceptor drums. Each exposure is developed by a dedicated development system and transferred, in registration, to a transfer image belt from which the transfer image is then transferred, in turn, to an output sheet and then fused. In yet another tandem embodiment, each developed image is successively transferred directly to a copy sheet being transported by a transfer belt. For example, U.S. Pat. Nos. 4,847,642; 4,912,491; and 4,916,547 disclose single pass color systems utilizing four separate exposure stations associated with four photoreceptor drums. Each exposure station includes a dedicated optical system consisting of a laser, polygon scanner and beam shaping optics. Each exposure is developed on an associated photoreceptor drum and transferred sequentially to a copy sheet passing along a common transfer station.
Ricoh publication 1-151370 discloses a tandem architecture where dual optical systems are symmetrically located 180.degree. apart from two separate polygons mounted on a common shaft. Thus, four beams are generated with two parallel beams being reflected from one side of the polygon and two beams from the other side.
Ricoh U.S. Pat. No. 4,761,046 discloses another tandem system which teaches four separate optical exposure stations utilizing a single holographic spinner member. The scanning beams are directed to different sectors of the spinner and diffracted therefrom to expose the associated photoreceptor drum.
In the prior art systems, if the beams from the imaging systems are directed at a zero angle of incidence, beam reflections back along the scanning path would result in interference and could lead to possible damage to the laser diode. It is known in the prior art to design the exposure system so that the beam striking the photoreceptor is directed at some small angle, typically one or two degrees. A problem with this design approach is the complexity of the design and the alignment and maintenance required to maintain this small angle of incidence. An associated problem in adjusting the beam tilt is the associated moving of the impinging point on the photoreceptor.
According to the present invention, multiple beams are generated by a plurality of ROS units located in a common housing and directed to the surface of a plurality of associated photoreceptor drums. The entire housing is offset with respect to the photoreceptor drums so that the scanning beams emerging from the housing are at right angles to the housing and along parallel paths, the paths being offset a small distance from the center line of the drum resulting in non-normal incidence of the beam on the drum surface. Beam reflection from the drum surface is not directed back along the scanning optical path, thereby alleviating the problem associated with the prior art. More particularly, the present invention relates to an imaging system for forming color image exposures on a plurality of photoreceptor drums rotating in the same direction at the same rotational speed, said system including:
a plurality of Raster Output Scanners (ROS) units, each ROS unit generating image-modulated scanning beams, wherein said scanning beams are transmitted along parallel optical paths to expose the surfaces of associated photoreceptor drums, the improvement wherein said scan beams being transmitted along said optical paths are incident at the surface of the associated photoreceptor drums at a non-normal angle of incidence .theta., said scan angle .theta. greater than 0.